The scientific and technological interest in miniaturized gas, humidity, chemical, temperature, and pressure sensor devices has grown in recent years. The need for such devices spans a wide range of industries and applications, such as the medical instrumentation, food and agriculture, paper, automotive, electric appliance, petrochemical, and semiconductor industries, as well as the military, in, for example, gas, humidity, chemical, temperature, and pressure sensing applications. The wide range of environments to which these devices may be exposed severely limits the candidate materials that may be used to build the devices. A number of gas, humidity, chemical, temperature, and pressure sensor devices have been developed and built for specific applications. However, none of these devices demonstrate a suitable combination of the desired robustness, sensitivity, selectivity, stability, size, simplicity, reproducibility, reliability, response time, resistance to contaminants, and longevity. Thus, what are still needed, in general, are multi-gas and vapor sensor devices, among other sensor devices, that exploit the high sensitivity of differential scanning nano and picocalorimetry microelectromechanical systems (MEMS) to heat flow and the unique properties of certain thin films and nano and picoparticles, including their high adsorption potential, high adsorption rate under optimized conditions, high desorption rate under optimized conditions, high chemical stability, and heat release associated with the physisorption of gas and vapor molecules.
Response time, mechanical strength, power consumption, and crosstalk between unit sensor devices are major areas of concern with respect to thermally-sensitive microelectromechanical systems (MEMS), such as gas, humidity, chemical, temperature, and pressure sensor devices, as well as calorimeter and microheater devices, in general. For example, faster response time provides higher sensitivity and greater mechanical strength provides higher reliability. Likewise, lower power consumption is desired for portable and wireless devices and less crosstalk between unit sensor devices provides greater accuracy. Response time and sensitivity are critical in many sensing applications, such as in sensing for warfare agents, measuring low dew points, detecting trace gases, etc., but are difficult to optimize with conventional multi-gas and vapor sensor devices without making sacrifices with respect to other performance parameters. Power consumption and crosstalk between unit sensor devices are both affected by thermal isolation. Typically, thermal isolation has been addressed by fabricating microelectromechanical systems (MEMS) on thin insulating membranes with low heat capacity. However, such thin membranes are fragile, resulting in low yield and reliability problems. Moreover, the peripheries of these thin membranes are typically bonded to a silicon substrate, introducing lateral heat conduction losses. Thus, what are needed are microelectromechanical systems (MEMS) that are built with, for example, low-thermal conductivity regions around the active thin membrane regions, resulting in more robust, high-performance, high-sensitivity microelectromechanical systems (MEMS).
Two additional areas of concern are raised with respect to miniaturized vapor (e.g., humidity) sensor devices, among other sensor devices. First, the polymeric sensing films associated with such vapor sensor devices often become significantly swollen while at relatively high humidity due to their high affinity for water vapor. The swelling of these sensing films generates lateral stresses that impinge upon the thin membranes, potentially breaking them. Second, sensing films having larger surface areas are desired in order to reduce the thickness of the sensing films at a given mass. Reducing the thickness of the sensing films and incorporating nanostructures (e.g., nano-spheres, nano-rods, nano-fibers, etc.) into the sensing materials decreases the diffusion time constant of the water adsorption/desorption, reducing the response time of the vapor sensor devices. Thus, what are needed are micro-machined vapor sensor devices, among other sensor devices, that utilize, for example, high-aspect ratio silicon microstructures etched adjacent to the thin membranes. These silicon microstructures would serve as stress relievers at varying vapor (e.g., humidity) levels and provide both large surface areas for the sensing films, increasing the sensitivity of the vapor sensor devices, and effective heat conduction paths to the microheaters also disposed adjacent to the thin membranes.